Scientists have installed an early warning system in China's most precious Buddhist caves, hoping to protect its centuries-old murals from rises in temperature, humidity and carbon dioxide density brought by tourists. The system, developed jointly by the Dunhuang Academy based in the northwestern Gansu Province and Zhejiang University, consists of thermometers, hygrometers and carbon dioxide detectors, said Fan Jinshi, curator of the Dunhuang Academy. "When any one of these indices reaches an alarm level, the system will beep and we will evacuate all tourists and close the grotto until the warning is lifted," said Fan. Trial operation of the system has begun in 10 of the 50 grottoes that open to visitors. Management of the grottoes will eventually impose an upper limit on the number of tourists based on the monitoring results. An average 3,000 to 5,000 tourists flock into the desert city daily in the peak season between May and October to see its thousands of Buddhist cave murals, and, in doing so, exhale harmful gas into the caves. Ventilation has long been a problem as 85 percent of the caves are less than 25 square meters, and experts warn the murals -- dating as far back as the Fourth Century -- are losing their hue due to unrestricted tourist numbers. Tourists to Dunhuang were encouraged to make online reservations beforehand at the Academy's website (www.dha.ac.cn), said Li Ping, director of the reception department at the Dunhuang Academy. Dating back to 336 AD, the Mogao Grottoes, also known as the Caves of 1,000 Buddhas, were listed as a world heritage site by UNESCO in 1987. Altogether 735 caves have been found and frescos on the inside walls cover an area of 45,000 square meters. The caves also hold 2,400 colored Buddha statues.